It is known to use fingerprints for checking a person's identity. In such a check, current data from a current fingerprint from the person whose identity is to be checked is compared with previously recorded reference fingerprint data (below sometimes only called reference data) for one or more persons.
If the check concerns a verification of the person's identity, the current data from the current fingerprint is compared only with reference data for the person who the person whose identity is to be checked pretends to be.
If the check concerns an identification of the person's identity, the current data is compared with reference data for at least two, but usually several, different persons to determine whether the current fingerprint originates from any of these persons.
Reference fingerprint data for a plurality of persons can be stored in a local database for an individual fingerprint checking system or in a central database for a plurality of checking systems. Alternatively, reference data for a person can be stored in a personal unit, such as a personal memory card or smart card, which the person uses when he or she wants to authenticate himself/herself, i.e. prove his/her identity by verification or identification.
In some types of storing units there may be a limited storage capacity. In such cases it is desirable to have a small amount of reference data.
The comparison between reference data and current data can be made either in the same physical unit as the one in which the reference data is stored, in the same physical unit as records and processes the current fingerprint or in some other unit.
If the comparison is made in a unit with a limited processor capacity, it is desirable that the comparison between current data and reference data can be made in a simple way. If either current data or reference data has to be transferred from one unit to another in connection with the comparison, it is desirable for the amount of data to be small so that the transfer time will be short.
In many applications, the user wants to be able to authenticate himself and get access to a protected object without delay. Then it is desirable that the current data can be generated quickly, that the transfer time, if any, is short and that the comparison can be made quickly.
The reference data can correspond to a complete fingerprint as recorded. However, only part of the information in the fingerprint is usually saved as reference data.
For instance, it is known to save as reference data information about specific features, also referred to as minutiae points, in the fingerprint. These specific features are usually of two predetermined types, viz. fingerprint ridge endings and fingerprint ridge bifurcations. For instance, coordinates for where these features are placed can be saved as reference data. When checking a person's identity, the relative location of features in a current fingerprint is determined and then compared with the relative location of the features in the reference data.
From, for instance, WO 01/84494 it is also known to save as reference data partial areas of an image of a reference fingerprint. When checking a person's identity, corresponding partial areas are found in a current fingerprint and then compared with the partial areas in the reference data. To allow the final identity check to be carried out on a smart card with a limited processor and memory capacity, a partial area is stored as a so-called public partial area in a public part of the reference data on the smart card. In the identity check, this public partial area is read to a computer unit, in which it is compared with a current fingerprint to determine in which position in relation to this the public partial area fits best. In this way, the reference data is aligned with the current fingerprint. In the public part of the reference data, also coordinates for how other partial areas, so-called private partial areas, are placed in relation to the public partial area are stored. When the current fingerprint has been aligned with the reference data, these coordinates can be used to determine which partial areas in the current image are to be selected and sent to the smart card for comparison with the private partial areas.
It is further known to form a finger code by first finding an objective reference point in a fingerprint, then dividing the fingerprint into sectors in relation to the reference point, then Gabor filtering the image with a number of Gabor filters and finally calculating the finger code as the variance in each sector of the Gabor-filtered image. The obtained finger code is advantageous as reference data since it will be quick and easy to compare a reference finger code with a current finger code. However, it takes relatively long to find the finger code. Another problem is that it is based on the fact that an objective reference point can be established in the fingerprint merely by searching in the same. It is known, however, that for certain fingerprints it is difficult, not to say impossible, to establish such an objective reference point.